A switched reluctance drive (Switched Reluctance Drive: SRD) system is the latest generation of stepless speed control system developed after a variable frequency speed control system and a brushless DC motor speed control system, and it is an optical, mechanical and electrical integrated high-tech integrating the modern microelectronics technology, the digital technology, the power electronics technology, the infrared photoelectric technology and the modern electromagnetic theory.
A switched reluctance motor speed control system is mainly composed of four parts: a switched reluctance motor (SRM), a power converter, a controller and a rotor position detector. The controller includes a control circuit and the power converter, and the rotor position detector is installed at one end of the motor.
The switched reluctance motor (SRM) used in the switched reluctance motor speed control system is a component that realizes electromechanical energy conversion in the SRD, and it is also the main sign of the SRD that is different from other motor drive systems. Salient poles of a stator and a rotor of a double-salient-pole variable reluctance motor of the existing SRM system are formed by laminating ordinary silicon steel sheets. The rotor has neither winding nor permanent magnet, a concentrated winding is twined on the stator pole, two radial opposite windings are connected together and are called “one phase”, the SR motor can be designed into a variety of different phase number structures, and the poles of the stator and the rotor have a variety of different combinations. A large number of phases and a small step angle are conducive to reducing the torque ripple, but the structure is complicated, and there are many main switching devices, the cost is high, and a four-phase (8/6) structure and a three-phase (12/8) structure are mainly applied at present.
A switched reluctance motor transmission system combines the advantages of an induction motor transmission system and a DC electric vehicle motor transmission system and is a powerful contender for these transmission systems. Its main advantages are as follows:
1. The switched reluctance motor has a larger motor utilization factor, which may be 1.2 to 1.4 times as large as an induction motor utilization factor. 2. The structure of the motor is simple, and no winding in any form is twined on the rotor; only the simple concentrated winding is twined on the stator, the end part is relatively short, and there is no inter-phase jumper. Therefore, the switched reluctance motor has the characteristics of less manufacturing procedures, low cost, reliable work, small maintenance load and the like. 3. The torque of the switched reluctance motor has nothing to do with the polarity of the current, only one-way current excitation is needed, only one switching element is used by each phase in a power conversion circuit ideally and is connected with a motor winding in series, thus having no feedthrough risk of two switching elements just like a PWM inverter power supply. Therefore, the switched reluctance motor drive system SED is simple in circuit and high in reliability, and has a lower cost than a PWM AC speed control system. 4. The structural form of the rotor of the switched reluctance motor has a small rotating speed limitation and can be made into a motor with a high rotating speed, furthermore, the rotational inertia of the rotor is small, and the size and the direction of a phase torque can be changed at any time when the phase of the current changes every time, so that the system has good dynamic response. 5. The SRD system can achieve mechanical characteristics satisfying different load requirements by controlling the turn-on and turn-off of the current and controlling the amplitude, it is easy to implement functions such as soft start, four-quadrant operation and the like of the system, and the control is flexible. Further, as the SRD system runs as a self-synchronizing system, it does not suffer from instability and oscillation problems at low frequencies just like a variable frequency power supply induction motor. 6. Since the SR switched reluctance motor adopts a unique structure, a design method and corresponding control techniques, the unit processing thereof can be comparable to that of the induction motor, and even has slight advantages. The efficiency and the power density of the SRD system can be maintained at a teaching level within a wide speed and load range.
The main disadvantages of the switched reluctance motor drive system are as follows:
1. There is torque ripple. It can be seen from the working principle that, the torque generated on the rotor of the S switched reluctance motor is formed by the superposition of a series of pulse torques, due to the influence of the double-salient-pole structure and the saturation nonlinearity of the magnetic circuit, the synthetic torque is not a constant torque, but has a certain harmonic component, which affects the low speed running performance of the SR motor; 2. The noise and vibration of the SR motor drive system are larger than those of a general motor.
The above disadvantages are essentially caused by the structure of the switched reluctance motor SRM of the switched reluctance motor drive system, that is, the SRD system. To reduce the torque ripple and the noise and vibration caused accordingly, the structure of the switched reluctance motor SRM must be changed.
A three-screw pump is formed by a pump body and screws, when a driving screw rotates, it drives a driven screw engaged with the driving screw to rotate together, a screw engagement space volume at one end of a suction cavity gradually increases, and the pressure decreases.
The Concept of the Three-Screw Pump
The liquid enters the engagement space volume under the action of a pressure difference. When the volume is increased to the maximum to form a sealed cavity, the liquid continuously moves in the sealed cavity along the axial direction until being discharged from one end of a discharge cavity. At this time, the screw engagement space volume at one end of the discharge cavity is gradually reduced to discharge the liquid. The working principle of the three-screw pump is similar to that of a gear pump, except that gears are replaced with the screws on the structure. The table shows the characteristics and the application ranges of various screw pumps. The three-screw pump has small flow and pressure pulses, little noise and vibration, and self-priming capability, but screw machining is difficult. The pump has two types of structures, namely, single suction and double suction, but a single-screw pump only has the single suction structure. The three-screw pump must be equipped with a safety valve (the single-screw pump does not have to be equipped) to prevent the output pressure of the pump from exceeding an admissible value due to some reasons, such as blockage of a discharge pipe, resulting in damage to the pump or a prime motor.
The Structure of the Three-Screw Pump
The three-screw pump suctions and discharges the liquid by using the rotation of the screws. The middle screw is the driving screw, which is driven by the prime motor to rotate, and the screws on the two sides are driven screws, which reversely rotate with the driving screw. The threads of the driving screw and the driven screws are double-ended threads.
The three-screw pump is a screw type positive displacement pump. In the three-screw pump, due to the mutual engagement of spiral grooves on the driving screw and the driven screws and their cooperation with inner surfaces of three holes of a bushing, several dynamic sealed chambers are formed between the inlet and the outlet of the pump, these dynamic sealed chambers continuously cause the liquid to move from the inlet to the outlet of the pump along the axial direction and cause the pressure of the conveyed liquid to gradually increase level by level. Therefore, continuous, smooth and axially moving pressure fluid is formed. The liquid conveyed by the three-screw pump is lubricating liquid free of solid particles, non-corrosive oil and substances similar to oil, the viscosity is 1.2-100 oE (3.0-760 cst), the high-viscosity liquid can also be conveyed after heating and viscosity reduction, and its temperature does not exceed 150° C.
Due to the mutual engagement of the screws and the close fit of the screws with the inner wall of the bushing, the space between a suction opening and a discharge opening of the pump is divided into one or more sealed spaces. With the rotation and engagement of the screws, these sealed spaces are continuously formed at the suction side of the pump to seal the liquid in the suction chamber, and the liquid is continuously pushed to the discharge end in the suction chamber along the axial direction of the screws, the liquid sealed in the spaces is continuously discharged, just like the situation that a nut is continuously pushed forward during the rotation of the threads, wherein the circles of the threads are seen as the liquid, when the screw rotates, the threads rotate, which is equivalent to the situation of the liquid in the screw pump, and this is the basic working principle of the screw pump.
From the characteristic that the driven screw is driven by the hydraulic force of the medium in the sealed chamber to rotate, it can be seen that the operation of the cycloid meshed three-screw pump is also reversible, moreover, since the screw diameter is small, the gyroscopic moment can be reduced, and the flywheel effect is very small, so that a full-load torque can be immediately generated during the startup, fast turning can be achieved, and the noise is small as well. The full-load continuous start (stopping and restarting) is performed on the high-pressure three-screw pump for more than 10 times per minute. When the pump is stopped, due to the back flow of the high pressure medium, the pump immediately rotates reversely, and the pump rotates reversely when the pump is restarted, the action is free, and the operation is reliable. Therefore, if the medium of a pressure source is input into the pump, the three-screw pump becomes a hydraulic motor. At this time, the three-screw pump becomes the prime motor. The only difference between the hydraulic motor and the three-screw pump lies in the interchange of the inlet and outlet. The hydraulic motor pumps the high-pressure medium into the pump, and the three-screw pump discharges the high-pressure medium from the pump, therefore the rotation directions thereof are reverse. The efficiency of such hydraulic motor will still remain high within a considerable load range. A pump, with a large helix angle of the screw, namely, a pump with a large helical pitch, is preferably used as the hydraulic motor.
As new energy technologies become more sophisticated, the drive system has achieved technological breakthrough. At present, the mainstream new energy vehicle models use parallel and series-parallel hybrid power systems. Through the analysis and distribution of engine and motor characteristics and efficiency ranges, the energy is efficiently used and recovered, thereby improving the overall system efficiency.
In the prior art, with the continuous advancement of science and technology, the new energy technology has become more widely used in public facilities. Currently, the new energy technology is widely used in public vehicles. However, the parallel and series-parallel structure is very complicated and is very troublesome to produce and manufacture.
Moreover, there are currently oil-liquid hybrid systems, oil-electric hybrid systems and electric-liquid hybrid systems, but no system in which hydraulic power generated by electricity is mixed with electricity is available. The existing motor and the hydraulic motor are separately arranged, the hydraulic motor and the hydraulic pump can be mixed for use, and the hydraulic motor and the motor are never mixed together.